Percutaneous transluminal coronary angioplasty (PTCA) is used to reduce arterial build-up of cholesterol fats or atherosclerotic plaque. Typically a guidewire is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the guidewire and a balloon catheter advanced within the guiding catheter over the guidewire. The balloon at the distal end of the catheter is inflated causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten re-closure of the dilated vessel or even perforations in the vessel wall. Implantation of a metal stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
An implanted prosthesis such as a stent can preclude additional procedures and maintain vascular patency by mechanically supporting dilated vessels to prevent vessel collapse. Stents can also be used to repair aneurysms, to support artificial vessels as liners of vessels or to repair dissections. Stents are suited to the treatment of any body lumen, including the vas deferens, ducts of the gallbladder, prostate gland, trachea, bronchus and liver or any other lumen of the body. The invention applies to acute and chronic closure or reclosure of body lumens.
A stent is a cylindrically shaped wire formed device intended to act as a permanent prosthesis. It has a plurality of metal elements joined to permit flexing of the cylindrical body along its longitudinal axis thereby conforming to a curved body lumen. A stent can be implanted during a single angioplasty procedure by using a balloon catheter bearing a stent which has been crimped onto the balloon. Stents radially expand as the balloon is inflated, forcing the stent into contact with the body lumen thereby forming a supporting relationship with the vessel walls. Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the balloon catheter.
Various shapes of stents are known in the art. U.S. Pat. No. 4,649,9221 to Wiktor for "Catheter Arrangement Having A Variable Diameter Tip and Spring Prothesis" discloses a linearly expandable spring-like stent. U.S. Pat. No. 4,886,062 to Wiktor for "Intravascular Radially Expandable Stent and Method of Implant" discloses a two-dimensional zig-zag form, typically a sinusoidal form. U.S. Pat. No. 4,969,458 to Wiktor for "Intracoronary Stent and Method of Simultaneous Angioplasty and Stent Implant" discloses a stent wire coiled into a limited number of turns wound in one direction then reversed and wound in the opposite direction with the same number of turns, then reversed again and so on until a desired length is obtained. U.S. Pat. No. 5,019,090 to Pinchuk for "Radially Expandable Endoprosthesis and the Like" discloses a plurality of adjacent generally circumferential sections that are substantially axially positioned with respect to each other. At least one of the generally circumferential sections has a generally circumferentially disposed expandable segment that imparts circumferential and radial expandability to the stent. U.S. Pat. No. 5,116,365 to Hillstead for a "Stent Apparatus and Method for Making" discloses a stent constructed from two elongated wires which are each bent into a series of tight bends. The two wires are permanently adhered at a first interconnection junction. The two wires are then wrapped around a mandrel repeatedly forming two opposing series of interconnections. U.S. Pat. No. 5,133,732 to Wiktor for "Intravascular Stent" discloses a stent body coiled from a generally continuous wire with a deformable zig-zag structure with a means for preventing the stent body from stretching along its longitudinal axis.
Stents have limited ability to provide effective patching of perforated vessels due to the spacing between metal elements. U.S. Pat. No. 4,878,906 to Lindeman et al. for "Endoprosthesis for Repairing a Damaged Vessel" describes an endoprosthesis made of a thin wall molded plastic sleeve intended to be collapsed radially and delivered to a damaged area of a vessel where it is expanded to provide a sealed interface to the vessel on its outer peripheral ends. The endoprosthesis therefore provides a patch which prevents leakage of blood from a vessel wall. The endoprosthesis disclosed employs various molded-in ribs, struts and the like to adapt the device for particular applications and to provide the desired degree of stiffness to form the sealed interface with the vessel wall. Such a stiff prosthesis, however, could not be expected to have the longitudinal flexibility needed to adapt to curved vessels.
In addition to limited ability to provide effective patching of perforated vessels due to the spacing between metal elements, such metal stents also have limited ability to carry and deliver drugs, such as antirestenosis drugs or anti-coagulant drugs, to the site of an intravascular injury. WO 91/12779 to Wolff et al. for "Intralumenal Drug Eluting Prosthesis" suggests that antiplatelet agents, anticoagulant agents, antimicrobial agents, antimetabolic agents and other drugs could be supplied in polymeric stents to reduce the incidence of restenosis. U.S. Pat. No. 5,282,823 to Schwartz et al. for "Intravascularly Radially Expandable Stent" discloses a plurality of metal elements joined to allow flexing of the cylindrical body along the longitudinal axis of the body and a polymeric film extending between the metal elements of the stent.